1. Field of the Invention
The present invention relates to the field of locking nuts, particularly two-piece locking nuts employing opposite handed threads on either nut.
2. Description of the Prior Art
Many different techniques have been used to prevent rotation of a threaded nut after it is secured. Wheel spindle assemblies often use castle nuts with cotter pins, where the spindle is threaded to receive a castle nut, and the spindle has a hole drilled through perpendicular to the axis of the spindle to receive a cotter pin. The cotter pin engages the castle nut and the spindle to prevent rotation of the castle nut. The castle nut design presents problems of an added operation to drill a hole through the spindle for the cotter pin, the difficulties with assembling a cotter pin including the rotational alignment of the nut to the hole in the spindle, insertion and deformation of the pin, and serviceability of the assembly.
A second locking nut design comprises a nut with a keyway machined into the shaft and nut, where a key is installed after the nut is threaded onto the shaft to prevent rotation of the nut to the shaft. This design requires additional machining operations of a key slot into the spindle and nut, plus the additional key component. The key design adds to the complexity of the assembly due to alignment of the key slots and installation of the key. Serviceability of the assembly is also more difficult.
Examples of 2-piece locking nuts which do not require additional parts include double nuts or double nuts with threads of differing pitch, but which use the same hand thread. The first of the same-hand double nuts is typically installed onto a male threaded bolt and tightened to create a clamp load against the part to be secured. The second nut is then installed and secured against the first nut to create a clamp load against the first nut. The reactive clamp load between the nuts produces a resistance to relative rotation of the nuts. This design may fail to act as a locking nut when the resistance to rotation between the first and second nuts is greater than the resistance to rotation between the nuts and the threads. In such a situation, both nuts may rotate simultaneously and the clamp load is lost between the first nut and workpiece, while the clamp load is maintained between the two nuts. Alternatively, if the clamp load between the two nuts is not great enough, the second nut may loosen and the locking nut is functionally now a simple single nut. To overcome this problem, the second nut may have deformable threads or use a chemical locking agent. These designs may be difficult to disassemble. Furthermore, the deformable nut is not reusable, and the chemical locking agent requires reapplication after removal for service.
A two-piece locking nut design which uses a different pitch on either nut includes a second nut with a finer thread than the first nut; thus the second nut must rotate more turns per unit of linear translation than the first nut. Due to the differing rates of rotation, the interference condition between the two nuts is maintained if the nuts rotate simultaneously, because the first nut travels further linearly than the second nut thereby increasing the interference between the nuts. Problems with these designs include the fact the second nut must often be secured to a high torque to create a suitable interference with the resulting resistance to rotation between the nuts, because of the small flat area of contact between the nuts. Additionally, if the frictional resistance from the interference between the first and second nuts is not as great as the frictional resistance between the first nut and the workpiece, the second nut may back off and the first nut will act as a simple single nut.
U.S. Pat. No. 1,473,321 ('321) discloses a design which uses a same-hand double nut locking nut design with varying pitch threads. To overcome the concern of the second nut backing off, '321 uses a beveled or flaired socket on the first nut and a beveled or flaired flange on the second nut which fits into the socket of the first nut. Additionally, '321 includes a malleable washer to grip the nuts at the interface to prevent corrosion and to restrain the second nut from backing off, as described in lines 96-98 of the '321 patent. The '321 design may experience the same problem described above, where the second nut may back off while the first nut is retained by the clamp load against the work piece, and result in a simple single nut system. '321 also requires an additional malleable washer to overcome the problem of the nuts becoming disengaged. In addition to the added expense and operation of installing the malleable washer, compression of the washer requires a work during assembly of the nuts; this work may require a higher torque and may result in a relaxed joint after the assembly is complete. The relaxed joint may then result in an ineffective locking nut.
U.S. Pat. No. 1,467,824 ('824) discloses a design which includes a bolt and two adjacent internally threaded members which have opposite hand threads and a conically tapered interface. The first internally threaded member of '824 is a difficult part to manufacture, consisting of a sleeve 13 with an internal thread and a head 15 formed at one end of the sleeve with an inset taper at both ends 14, 16 of the sleeve. The first end 14 of the sleeve clamps against a conical mating portion 12 near the head of the bolt. The side of the head at the opposite end of the sleeve clamps against the work piece, as described at lines 106-109. The second internally threaded member is a conically-shaped 18 nut 17 which clamps against the inset conical tapered head 16 of the sleeve, opposite the workpiece. The locking nut feature of '824 requires a conical interface between the sleeve and the bolt head, as described on the second page of the specification at lines 15-19. Problems with this design include the inability to clamp varying thickness workpieces, because the thickness of the workpiece is dictated by the distance between the first end of the sleeve which clamps against the bolt and the head of the sleeve which clamps against the workpiece. The '824 design thus requires the length of the threaded sleeve and workpiece to be precise to create a clamp load. The sleeve is a complicated part to manufacture, as it has a long internal thread, a hex head at one end, and an internal conical surface at both ends. The length of the sleeve dictates how far the sleeve must be threaded during installation. Thus a long sleeve will require numerous rotations to secure against the head of the bolt, requiring time at assembly and disassembly.
It would be desirable to have a two-piece locking nut design which is simple to manufacture, assemble, and service, and which accommodates varying thickness workpieces, plus overcome the problem of simultaneous loosening of the nuts.